[travis] Unconditionally build simbody with clang
[travis] Unconditionally build simbody with clang

Even if building opensim with gcc, build simbody with clang. To reduce build time.

Also, remove the SIMBODY_STANDARD_11 cmake variable, which is now gone from simbody.
opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core,opensim-org/opensim-core